Herb was born on February 14 (Valentine's Day) in an unpecified year during the 1970s (aged 40s). He was a nerd in high school and was often called "Big Bird", due to his height of 203 centimeters and his tendency to wear yellow jerseys. He never mentioned having his first date until college, where he claimed to have stopped wearing yellow shirts and lowered his standards. He also told Alan that he lost his virginity to one of Alan's girlfriends on a driveway, and reveals that "[they] weren't even in a car." He was married once and he and his wife tried to have kids, but his wife passed away and in Judith and Herb's early relationship, Alan mentioned "No alimony. Got to love that." and Herb just gave Alan a look. Herb was Jake's pediatrician when he met Judith, and after she divorced Alan, they dated in the second season. They broke up, but got back together in season four, and got married. Alan at first didn't like the idea, until he learned he wouldn't be paying alimony anymore. The wedding was in many ways a disaster, days before Charlie slept with Myra, Herb's sister and grew to genuinely like her, though she broke up with him because she was engaged, and later Herb's ex-fiance Shannon locked Alan in a closet with her and kept shouting his name all through the ceremony to make it appear they were having sex.
Herb's marriage to Judith was very much like Alan's marriage to her, often leading Alan to give Herb tips on what to do. Although he sincerely loves Judith, their marriage is not perfect, and, on one occasion, Herb inadvertently reveals to Alan that he sleeps in the guest room, which is why he uses the guest towels. He also, during a discussion with Charlie, Alan, Gordon, and Chris, in Lyndsey's house after Alan burns it down, Herb reveals that he has a "non-complicated, no-lies marriage", to which Charlie says: "You've been taking out the garbage for 2 hours, Herb." The reason for Herb's implied marital issues may come from his goofy, buffoonish personality, and his lack of common sense, which leads him to advise Alan to use tomato juice and ketchup when the latter is sprayed by a skunk.
Alan and Herb are in fact good friends, much to Judith's dismay. Herb even allows Alan to move in with him and Judith after a fight with Charlie, and even gets Alan's stuff before deciding to hang out at Charlie's. Whenever he and Judith have a fight, Herb tends to hang out at Charlie's house, which usually annoys Charlie. Herb has, on occasion, shown a certain envy of Charlie's single lifestyle, and describes Charlie as "a little loosie-goosie with the liquor and the ladies, but all-in-all a good fella", and is also envious of Alan's former relationship with 22 year-old Kandi. He has also shown a (not entirely) subtle attraction to Lyndsey, Alan's girlfriend, and when Alan confides in him his suspicion that Charlie and Lyndsey are having an affair, Herb says: "That lucky-ducky!" Alan also reveals to Lyndsey that whenever Herb looks at her, he is imagining her wet and naked.
When Herb was kicked out by Judith for an extended period of time, he hired prostitutes for himself and Charlie, however he felt so ashamed before doing it he went running back to Judith. Judith and Herb had sex and Judith got pregnant, though unbeknownst to Herb she also had unprotected sex with Alan, meaning either could be the father. When Judith was in labor, Herb broke his leg, and had to be treated himself, leaving Charlie to be Judith's coach. Herb and Judith named their daughter Mildred, although she is more commonly called "Milly". Herb has been described as "the best Judith's ever had." Herb is also on a water polo team and in a barbershop quartet, and his hobbies revolve around gardening, a past-time about which he connotes sexually, saying that when he plants seeds, he makes sure the soil is moist by "sticking his finger in the ol' mother earth", and, if it comes up dry, he "whips out my hose and gives it a good spritz", before "planting his seed in the soil", and advises Alan and Charlie not to "fling that stuff around or half of it gets wasted." He also enjoys model trains, and owns an entire train-set.
In the Season 9 premiere, Herb appeared in two scenes during and after the funeral of Charlie Harper. He was saddened by the loss of his good friend, Charlie Harper, who gave him advice for 5 years prior to his unexpected death. He has appeared in four episodes in Season 9. He reappeared in Season 9 finale Oh Look! Al-Qaeda!.
In Season 10, Judith divorces Herb for cheating on her with his receptionist. She comes to collect him after he becomes depressed, throws parties at Walden's beachhouse, binge drinks, loses his mind, and moves into a singles apartment. They later reconcile, however Herb apparently hasn't gotten over his philandering ways, with Walden finding out he has hired another hot receptionist after reconciling with Judith.
Herb returns in the fourth episode of Season 12 called Thirty-Eight, Sixty-Two, Thirty-Eight to celebrate Alan and Walden adopting the soon-to-be born baby of Kathy Bates. In Bouncy, Bouncy, Bouncy, Lyndsey, he invites Alan over to his yoga class in order to stay fit. One of the students there turns out to be Lyndsey McElroy. After Alan argues with her, Herb expresses interest in her but assures Alan he'll stay away from her. This however turns out to be a lie.
- He is oblivious to Charlie's sex references when he is gardening, but the way he gardens helps his sex life.
- The character was named Greg in early Season 2 episodes, however when he became a recurring character in Season 4 his first name was changed to Herb. It can be assumed that Greg is his middle name.
- Despite loving Judith, she seems to treat him in the same cruel and unfair way she did to Alan.
- He loves playing with model trains, plays waterpolo and sings in a barbershop quartet (The Two Finger Rule). He also enjoys gardening.
- During high school he was nicknamed "Big Bird" because of his height and habit of wearing yellow sweaters.
- Herb and Judith have both met and become good friends with Walden Schmidt, who bought Charlie's beach house after his death and lets Alan and Jake continue to live there.
- Herb does not know about Judith's brief affairs. Her first was with Alan, when he was kicked out of the house in Season 6. The second time was in Season 9 with Walden when Judith went to talk sense into him about his influence on Jake, after Walden had told Jake he had dropped out of school and started his own business, but instead made a move on him.
- As of Season 9, Herb Melnick and Walden Schmidt are the only people who consider Alan to be a friend of theirs.
- Herb's marriage to Judith was half the length of her marriage to her first husband, Alan.
- Although it's clearly stated he is a pediatrician, in season 12 Herb treats Alan for a pulled neck muscle and gives him a prostate exam.